Vibrating meters, such as for example, vibrating densitometers and Coriolis flow meters are generally known and are used to measure mass flow and other information for materials within a conduit. The meter comprises a sensor assembly and an electronics portion. The material within the sensor assembly may be flowing or stationary. Each type of sensor may have unique characteristics, which a meter must account for in order to achieve optimum performance.
Exemplary Coriolis flow meters are disclosed in U.S. Pat. No. 4,109,524, U.S. Pat. No. 4,491,025, and Re. 31,450 all to J. E. Smith et al. These flow meters have one or more conduits of straight or curved configuration. Each conduit configuration in a Coriolis mass flow meter has a set of natural vibration modes, which may be of simple bending, torsional, or coupled type. Each conduit can be driven to oscillate at a preferred mode.
Material flows into the flow meter sensor assembly from a connected pipeline on the inlet side of the sensor, is directed through the conduit(s), and exits the sensor through the outlet side of the sensor. The natural vibration modes of the vibrating material filled system are defined in part by the combined mass of the conduits and the material flowing within the conduits.
When there is no flow through the sensor assembly, a driving force applied to the conduit(s) causes all points along the conduit(s) to oscillate with identical phase or small “zero offset,” which is a time delay measured at zero flow. As material begins to flow through the sensor assembly, Coriolis forces cause each point along the conduit(s) to have a different phase. For example, the phase at the inlet end of the sensor lags the phase at the centralized driver position, while the phase at the outlet leads the phase at the centralized driver position. Pick-off sensors on the conduit(s) produce sinusoidal signals representative of the motion of the conduit(s). Signals output from the pick-off sensors are processed to determine the phase difference between the pick-off sensors. The phase difference between the two or more pick-off sensors is proportional to the mass flow rate of material flowing through the conduit(s).
The mass flow rate of the material can be determined by multiplying the phase difference by a Flow Calibration Factor (FCF). Prior to installation of the sensor assembly of the flow meter into a pipeline, the FCF is determined by a calibration process. In the calibration process, a fluid is passed through the flow tube at a known flow rate and the relationship between the phase difference and the flow rate is calculated (i.e., the FCF). The sensor assembly of the flow meter subsequently determines a flow rate by multiplying the FCF by the phase difference of the pick-off sensors. In addition, other calibration factors can be taken into account in determining the flow rate.
Due, in part, to the high accuracy of vibrating meters, and Coriolis flow meters in particular, vibrating meters have seen success in a wide variety of industries. One industry that has faced increased demands for accuracy and repeatability in measurements is the oil and gas industry. With the increasing costs associated with oil and gas, custody transfer situations have demanded improvements in measuring the quantity of oil that is actually transferred. An example of a custody transfer situation is fuel bunkering. Bunkering refers to the practice of storing and transferring marine fuel oils, which have come to be known as bunker fuels. For ship fueling, large amounts of fuel may be temporarily stored in a barge or other container for the purpose of transferring fuel from shore to a ship. A bunker may be located on a dock or other port facility, or may be carried by a barge or other refueling vehicle. During bunkering, the fuel measurement usually comprises an empty-full-empty batching process, thereby allowing gas to become entrained in the fuel. The entrained gas in the fuel produces serious measurement difficulties as both the volume and the mass of the fuel being delivered changes. Additionally, at the beginning and the end of the process, the flow meter may be partially filled with fluid rather than completely empty or completely full.
Bunker fuel comprises a relatively heavy petroleum derivative that is used in heating or in large industrial and/or marine engines. There are multiple grades of fuel that may comprise a bunker fuel. Bunker fuel is generally heavier and more viscous than gasoline or diesel.
Marine fuel costs represent a major portion of a ship's operating cost. With increasing oil prices and increasing conservation efforts, careful fuel management has become vital for environmental and financial reasons.
Improvements in Coriolis flow meters have made it possible to obtain more accurate measurements of fuel even with entrained gas. A problem can exist however, whenever flow is stopped, for example at the beginning or at the end of the bunkering process when the valves and pumps delivering the fuel are closed. One reason is due to a change in the zero offset of the vibrating meter. Even after fuel has stopped flowing through the Coriolis flow meter, the flow tubes continue to vibrate. Ideally, the time delay between the pick-off sensors would return to the original zero offset value when the flow through the tubes is zero. As long as the time delay returns to the original zero offset, the Coriolis flow meter will report a zero mass flow. However, various factors attribute to the zero offset of the sensor assembly and some of the factors may change either during the bunkering process or after the last zeroing process.
For example, while many Coriolis flow meters are capable of maintaining accurate measurements despite entrained gas, in some situations when the flow through the flow tubes falls to zero, the entrained gas can lead to an imbalance that creates asymmetric damping between the inlet and the outlet side of the vibrating meter's sensor assembly. The asymmetric damping can cause a time delay between pick-offs, which may be different than the original zero offset and thus may be interpreted as real flow. This problem may also be experienced if the sensor assembly is only partially filled with fluid, for example. Even in vibrating meters that are tolerant of fuel including entrained gas, it may be desirable in some situations to cease measuring flow through the Coriolis flow meter after the valves and pumps have been closed or shut off. This is because the fuel within the pipeline that continues to flow downstream of the fluid control valve due to residual pressure or gravity, for example, may have been in the system already. Thus, fuel previously within the system should not be counted towards the bunkering total. Often coordinating the totalizer of the flow meter with the closing of the valves and pumps is difficult because the flow meter may not be in communication with the valves or the pump.
Various prior art methods have been proposed to deal with the false readings associated with aerated fluid within the flow tubes at zero flow. One of the most frequently used methods is to simply read the totalizer value immediately after turning off the pump and/or closing the valve. The idea behind this method is to obtain a reading before the vibrating meter is able to output false measurements. However, this approach relies upon operator intervention to closely monitor the situation. This approach also assumes that the operator reading the totalized value wants the meter to measure a proper batch total.
Another method is to increase the low flow cutoff of the vibrating meter. The low flow cutoff is a value that assumes time delays below the low flow cutoff are due to the zero offset and thus, equal zero flow. A problem with this method is that as the low flow cutoff value increases, there is an increased potential of real flow values being below the low flow cutoff. Therefore, this approach can result in real flow values being forced to zero.
Yet another prior art approach is to set high and low density limits so that totalizing is prevented if the density deviates from the known liquid density by more than a threshold amount. Unfortunately, this method does not work well in practice because the presence of entrained gas and solid particulates can cause positive and negative errors, which often cancel out. Additionally, in bunkering applications, considerable mass flow can actually occur when the measured density is below the known liquid density, for example, if there is 5% gas volume fraction during a portion of the batch delivery.
Consequently, these prior art approaches are inadequate in most situations. Therefore, there is a need in the art to provide an increased reliability and accuracy for vibrating meters. There is a need in the art to accurately determine when a vibrating meter should stop totalizing flow measurements. These and other problems are solved and an advance in the art is achieved.
The embodiments described below provide a system and method that substantially prevents false measurements in a vibrating meter. The embodiments described below provide one or more fluid switches proximate the vibrating meter. The fluid switches can detect certain flow conditions, such as the presence of fluid and/or fluid flow proximate the vibrating meter in order to determine if the vibrating meter should be totalizing measurements.